Many applications, including medical therapy and diagnostics, semiconductor processing and industrial radiography require beams of particles to be directed to particular positions with good accuracy, and with repeatable and timely control. A particular application where accurate scanning and positioning of high energy particle beams may be required is particle therapy. For particle therapy, beams of high energy charged particles, most often protons, but also heavier ions such as ionized carbon, oxygen and argon, may be used to deliver a therapeutic dose. Particle therapy may offers improvements over more conventional X-ray therapy by being able to deliver a dose much more precisely to a region within the body and with reduced unwanted damage to healthy tissues surrounding the region.
A method of particle beam therapy providing precise control and the ability to deliver a dose to the most complex volumetric shapes is pencil beam scanning. For pencil beam scanning, a narrow beam of mono-energetic particles may be deflected by controlled amounts so as to describe a pattern in space. For pencil beam scanning, angular deflection is typically less than ten degrees. In combination with modulation of the beam intensity and sequential delivery of patterns at different beam energies, a desired dose distribution may be achieved. Several such exposures may be performed over a period of days or weeks in order to complete a treatment plan.
A component of a pencil beam scanning system is the electromagnets that deflect the beam to the desired trajectory. These electromagnets may require timely magnetic field changes in order to develop a desired pattern without experiencing undue periods of time for the magnetic field to settle. These electromagnets may be required to deliver good ion optical qualities over the scanned portions in order to avoid disruption of the beam shape. These electromagnets should not occupy excessive space in the trajectory of the beam, as this may translate into a larger system, potentially higher costs, and may preclude the installation of systems in some locations. In order to support a broad array of treatments, the electromagnets should not impose arbitrary constraints for how the beam trajectory may be manipulated.
FIG. 1 presents an example illustration of a conventional electromagnetic apparatus for deflecting a charged particle beam.
A deflecting mechanism 100 includes a horizontal electromagnetic portion 102 and a vertical electromagnetic portion 104. The terms horizontal and vertical are used for convenience only and do not represent actual positions.
Deflecting mechanism 100 may operate to deflect the trajectory of a charged particle 106 in the horizontal direction via horizontal electromagnetic portion 102 and in the vertical direction via vertical electromagnetic portion 104. Not shown is a typical yoke for returning magnetic flux from portions 102 and 104.
Horizontal electromagnetic portion 102 may operate to generate a magnetic field in the vertical direction and vertical electromagnetic portion 104 may operate to generate a magnetic field in the horizontal direction.
Charged particle 106 may initially be moving in the direction of an axis 108. After transitioning through horizontal electromagnetic portion 102 and vertical electromagnetic portion 104, charged particle 106 may be moving in a different trajectory as denoted by a trajectory 110.
High quality dipole fields, with minimal higher order components, may be established via simple designs as illustrated in FIG. 1. Furthermore, beam aberrations introduced by the electromagnetic portions may be considered small. The operation of the two electromagnetic portions may be distinct. For example, horizontal electromagnetic portion 102 may have an air gap 112 where the magnetic field generated may be slightly greater than the dimension of the received beam of particles. In contrast, vertical electromagnetic portion 104 may have an air gap 114 which has a larger separation distance than exhibited by air gap 112 in order to accommodate the range of deflections generated by horizontal electromagnetic portion 102. Furthermore, the increased separation distance require for air gap 114 may require additional amp-turns for an energizing circuit 116 and may translate into a slower beam movement in the vertical direction generated via vertical electromagnetic portion 104. The increased air gap and increased amp-turns may result in more complexity for planning the map of potential beam positions due to differing speed of response of the in the horizontal and vertical axes. Furthermore, the conventional deflection apparatus, as illustrated in FIG. 1, may require an apparatus occupying increased space, which may be considered a premium for many systems.
FIG. 2 presents an example illustration of a conventional method and means for deflecting a charged particle using an electromagnetic apparatus.
A deflecting mechanism 200 includes a horizontal electromagnetic portion 202 and a vertical electromagnetic portion 204.
Deflecting mechanism 200 may operate to deflect the trajectory of a charged particle 206 in the horizontal direction via horizontal electromagnetic portion 202 and in the vertical direction via vertical electromagnetic portion 204. Horizontal electromagnetic portion 202 and vertical electromagnetic portion 204 may be configured as a quadrupole structure.
Horizontal electromagnetic portion 202 may operate to generate a magnetic field in the vertical direction and vertical electromagnetic portion 204 may operate to generate a magnetic field in the horizontal direction.
Charged particle 206 may initially be moving in the direction of an axis 208. After transitioning through horizontal electromagnetic portion 202 and vertical electromagnetic portion 204, charged particle 206 may be moving in a different trajectory as denoted by a trajectory 210.
The physical size for a two dipole design as illustrated in FIG. 1 may be reduced by superimposing the vertical and horizontal electromagnetic portions to create a quadrupole structure as illustrated in FIG. 2. The excitation of the electromagnetic portions for the quadrupole as illustrated in FIG. 2 may be dissimilar from that of a conventional beam focusing quadrupole. A conventional beam focusing quadrupole may be configured with four coils and a single power supply, with the direction of the current flow through the coils arranged to generate a zero magnetic field on the central axis of the magnetic air gap and a linearly increasing magnetic field with increased displacement from the central axis to shape the beam cross-section.
The superimposed dipole as illustrated with reference to FIG. 2 may be configured with two independent power supplies with one power supply associated with an opposed electromagnetic portion. The resultant magnetic field for the superimposed dipole may be considered as a vector sum of the fields of the two individual dipoles associated with the composite structure. The superimposed deflection apparatus may be controlled similar to two independent dipoles with one deflecting in the horizontal direction and one deflecting in the vertical direction. A square configuration for the electromagnetic portions is common, as other structures and configurations may result in a poor quality dipole magnetic field associated with the central axis and may also result in large pole spacing. However, even a square configuration for the superimposed dipole may result in a magnetic field which may be of considerably less quality than realized with equivalent separate dipoles as illustrated with reference to FIG. 1. Furthermore, as a result of the less quality magnetic field generated by the superimposed dipole, beam aberrations may be experienced.
In view of the foregoing, there is a need for improved techniques for electromagnets associated with deflecting charged particle beams.
Unless otherwise indicated illustrations in the figures are not necessarily drawn to scale.